enroll: add PEX sub-protocol to support enrolling new nodes into a network

enroll: add PEX sub-protocol to support enrolling new nodes into a network

This protocol does a full DH exchange and allows both sides to confirm
the result based on a session id hash derived from the DH session key.
It exchanges the public auth_key for a network and a newly generated keypair
for the added node

Signed-off-by: Felix Fietkau <nbd@nbd.name>